Many types of input devices may be used to provide input to computing devices, such as buttons or keys, mice, trackballs, joysticks, touch screens and the like. Touch screens, in particular, are becoming increasingly popular because of their ease and versatility of operation. Typically touch screens on interfaces can include a touch sensor panel, which may be a clear panel with a touch-sensitive surface, and a display device that can be positioned behind the panel so that the touch-sensitive surface substantially covers the viewable area of the display device. Touch screens allow a user to provide various types of input to the computing device by touching the touch sensor panel using a finger, stylus, or other object at a location dictated by a user interface being displayed by the display device. In general, touch screens can recognize a touch event and the position of the touch event on the touch sensor panel, and the computing system can then interpret the touch event in accordance with the display appearing at the time of the touch event, and thereafter can perform one or more actions based on the touch event.
Touch sensor panels can be formed from a matrix of row and column traces, with sensors or pixels present where the rows and columns cross over each other while being separated by a dielectric material. Each row can be driven by a stimulation signal, and touch locations can be identified through changes in the stimulation signal. Typically, a touch location is sensed based on an interference of the stimulation signal, such that a touch location may correspond to a location where the stimulation signal is the weakest. In some instances it may be desirable for a user to provide input to the touch screen with an input device other than the user's finger or fingers. Some input devices, such as styli, allow a user to use the input device as a pen or pencil and “write” on the touch screen. However, depending on the mode of operation of the touch sensor panels, the computing device may not be able to detect certain characteristics of the input stimulation provided by the user through the stylus. For example, in a capacitive-sensing touch screen, the touch sensor panels may not be able to detect how much force (e.g., pressure) is exerted by the user on the touch screen through the stylus and instead may only be able to detect the presence or absence of the stylus. The user may thus not be able to “write” on the touch screen with as much control as the user would be able to write on paper with a ballpoint pen or other non-electronic writing tool because, for example, the thickness of the lines drawn on a touch screen by the stylus will be uniform regardless of the force exerted by the user.